Live attenuated vesicular stonnatitis virus (VSV) was developed as a vaccine vector in our laboratory and is a potent vector in a variety of animal models of disease. Rhesus macaques vaccinated with VSV-based vectors are protected from AIDS following challenge with a hybrid simian-human immunodeficiency virus (SHIV). This protection is likely due in part to neutralizing antibody because the protected animals maintain virus-specific neutralizing antibody and T-cells while controlling virus to undetectable levels for at least five years. Although VSV vectors expressing specific HIV Env proteins can induce HIV neutralizing antibody, they fail to induce broadly neutralizing antibodies able to neutralize diverse strains. Because of high sequence diversity among HIV strains and the ability of the virus to evolve rapidly in response to host immune pressure, it is critical to test new approaches for eliciting potent and broadly neutralizing antibodies in animal models of AIDS. We propose to test two new approaches to induce neutralizing antibodies in an SIV model of AIDS. We will target two conserved regions of Env: the gp41 subunit and the CD4-inducible (CD4i) epitopes of gp120. To target gp41, we have generated recombinant VSV vectors that express a chimeric influenza HA1-SIV gp41 protein that is efficiently expressed on the cell surface. This molecule can potentially expose epitopes in gp41 that are normally hidden by gp120 and induce novel or higher levels of neutralizing antibodies directed at gp41. To maximize exposure of CD4i epitopes, we will generate recombinant VSV vectors that simultaneously express high levels of both CD4 and SIV Env. This approach should allow for very high levels of CD4-Env interactions on cells and on virus particles in vaccinated animals leading to extensive exposure of CD4i epitopes. The ability of these approaches to generate SIV neutralizing antibodies in serum and mucosal sites will be examined initially in small animals and later in rhesus macaques. By providing insight into how to enhance broadly neutralizing antibody responses to Env, the study may ultimately contribute to an effective HIV vaccine